Delay logging about missing certifi until it is really needed.

client/isolate_storage.py

 #!/usr/bin/env python
 # Copyright 2016 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 """A low-level blob storage/retrieval interface to the Isolate server"""
 
 import base64
-import binascii
 import collections
 import logging
 import os
 import re
 import sys
 import threading
 import time
 import types
 import uuid
 
@@ skipping 12 matching lines @@
   from google.auth.transport import requests as google_auth_transport_requests
   from proto import bytestream_pb2
 except ImportError as err:
   grpc = None
   bytestream_pb2 = None
 
 # If gRPC is installed, at least give a warning if certifi is not. This is not
 # actually used anywhere in this module, but if certifi is missing,
 # google.auth.transport will fail with
 # https://stackoverflow.com/questions/24973326
+certifi = None
 if grpc is not None:
   try:
     import certifi
-  except ImportError as err:
-    logging.warning('could not import certifi; gRPC HTTPS connections may fail')
+  except ImportError:
+    # Could not import certifi; gRPC HTTPS connections may fail. This will be
+    # logged in IsolateServerGrpc.__init__, since the logger is not configured
+    # during the import time.
+    pass
 
 # Chunk size to use when reading from network stream.
 NET_IO_FILE_CHUNK = 16 * 1024
 
 
 # Read timeout in seconds for downloads from isolate storage. If there's no
 # response from the server within this timeout whole download will be aborted.
 DOWNLOAD_READ_TIMEOUT = 60
 
 
@@ skipping 466 matching lines @@
 class IsolateServerGrpc(StorageApi):
   """StorageApi implementation that downloads and uploads to a gRPC service.
 
   Limitations: only works for the default-gzip namespace, and with zero offsets
   while fetching.
   """
 
   def __init__(self, server, namespace, proxy):
     super(IsolateServerGrpc, self).__init__()
     logging.info('Using gRPC for Isolate')
+    if not certifi:
+      logging.warning(
+          'Could not import certifi; gRPC HTTPS connections may fail')
     self._server = server
     self._lock = threading.Lock()
     self._memory_use = 0
     self._num_pushes = 0
     self._already_exists = 0
 
     # Proxies only support the default-gzip namespace for now.
     # TODO(aludwin): support other namespaces if necessary
     assert namespace == 'default-gzip'
     self._namespace = namespace
@@ skipping 173 matching lines @@
     namespace: isolate namespace to operate in, also defines hashing and
         compression scheme used, i.e. namespace names that end with '-gzip'
         store compressed data.
 
   Returns:
     Instance of StorageApi subclass.
   """
   if _grpc_proxy is not None:
     return IsolateServerGrpc(url, namespace, _grpc_proxy)
   return IsolateServer(url, namespace)